This invention relates to an optical device, and more particularly to an optical device where optical elements such as light-receiving and light-emitting elements are mounted in a header having a capacitor imbedded therein.
A conventional light-receiving element with a signal amplifying element is one of a type wherein a mount plate is disposed on an upper portion of a cylindrical and metal frame body. Further, leads are welded to the mount plate. Then, an eyelet and the mount plate are held together by a carbon jig and glass is melted under a high temperature and poured into the frame body. Thereafter, the glass poured into the frame body is sealed and fixed.
Each of the frame body, the mount plate and the leads is substantially identical in expansion coefficient to the glass.
A PIN photo diode (which corresponds to a diode serving as a light-receiving element wherein an I layer is interposed between P and N layers), an amplifying element (amplifier) and a power-noise removing chip capacitor are mounted on the mount plate by bonding. These components are electrically connected to each other by aluminum wires.
FIG. 5 shows an optical device 50 which is coupled to an optical fiber 41 with a holder 43. The optical fiber 41 is fixed in the center of the holder 43 with a cylindrical case 42 which is fixed by adhesive material 44. The optical device 50 is fixed into a recess portion of the holder 43 by adhesive material 45. A PIN photo diode (not shown in FIG. 5) is mounted on the center of the optical device 50. Since the recess portion of the holder 43 is formed so that a cap (a head portion) of the optical device 50 can be inserted without alignment, the optical fiber 41 and the PIN photo diode are optically aligned.
Although the PIN photo diode is mounted on the center of a header of the optical device, the center of the header may not be coincident with the center of the cap because there is some clearance between them.
Further, when the eyelet and the mount plate are held together by the carbon jig and the glass is molten under the high temperature and poured into the frame body, expansion/shrinkage occurs due to the difference in thermal expansion coefficient (carbon: a=3.3.times.10.sup.-6 and glass: a=48.times.10.sup.-7) between the carbon jig, the eyelet and the mount plate, thereby causing a positional displacement of the mount plate. When the PIN photo diode is mounted to the mount plate in a state in which the positional displacement has occurred in the mount plate, a problem arises that a displacement or shift of about .+-.0.2 mm occurs in the PIN photo diode.
In addition, since the distance between the above components which are to be connected to each other by the aluminum wires is long, a parasitic LC oscillating circuit is made up of the total capacitance (C) of an optical device and the total reactance (L) of the aluminum wires, so that the amplifier oscillates.